1. Field of the Invention
The present invention relates to an optical device which employs a composite comprising a liquid crystalline substance and an orientation-sustaining material such as a selected polymer or flat particulates capable of sustaining the orientation of the liquid crystalline substance. This optical device is provided with a memory function, and is characterized by the fact that liquid crystal orientation is sustained until electrical signals are applied to cancel it. The optical device can be used for light adjustment, displays, and data storage.
2. Description of the Prior Art The technology for fabricating liquid crystal/polymer composites comprising transparent polymer materials which have closed or open cells and a liquid crystalline substance which fills these cells is generally known. It is also generally known that such composites can be used in optical devices. The light-reflecting characteristics and light-transmitting characteristics of such composites change depending on the state of the liquid crystalline substance. For example, the composite may be used for light adjustment by switching its state between a transparent state and a light-scattering state. It is also possible to display a pattern by locally changing light reflection or light transmission in a prescribed area. The composite can also be used for data storage by combining it with readout technology.
It is common practice to employ an electric field or external force in order to switch the state of liquid crystal orientation. In ordinary optical devices employing a liquid crystalline substance, when the electric field or external force ceases to be applied, the liquid crystal orientation returns from the state prevailing during application of the electric field or external force to the state of orientation prevailing prior to application of the electric field or external force. On the other hand, some devices have a feature that the state of orientation under electric field or external force is sustained and does not return to the state of orientation prevailing prior to application of the electric field or external force even after the removal of the electric field or external force. In this specification, this sustaining characteristic is termed as "memory" or "memory function".
Japanese Laid-Open Patent Application 5-34668 discloses a technology for endowing liquid crystal/polymer composites with memory. In this technique, a compound that has hydroxyl groups is introduced into a polymeric material. The hydroxyl groups interact with the liquid crystal molecules in such a way that the state of orientation prevailing during application of the electric field is sustained even after the electric field has ceased to be applied, thus endowing the composite with memory. In other words, the hydroxyl groups of the polymeric material which encloses the liquid crystalline substance endow the liquid crystalline substance with memory, whereby the state of liquid crystal orientation prevailing during application of the electric field is sustained even after the electric field has ceased to be applied.
When a composite is endowed with memory, it is desirable to also make erasure possible. In the technique disclosed in Japanese Laid-Open Patent Application 5-34668, the liquid crystal/polymer composite is heated in order to return it from the state of orientation prevailing during application of the electric field (the state of orientation sustained by the hydroxyl groups) to the state prevailing prior to application of the electric field. In this Specification, the action of returning the state of liquid crystal orientation to the state prevailing prior to application of the electric field or external force is termed "erasure". In the technique disclosed in Japanese Laid-Open Patent Application 5-34668, erasure is effected through heating.
It is generally difficult to incorporate heating devices having relatively high power requirements into ordinary optical devices employing liquid crystalline substances. Even where possible, this raises the cost of the device, or increases the power consumption of the device.
Japanese Laid-Open Patent Application 63-294523 discloses a completely different technique for endowing an optical device with memory. In this technique, a liquid crystal display comprising a laminate consisting of a transparent surface protection film that can be deformed by external force, a liquid crystal layer, and a substrate is proposed. In this liquid crystal display device, writing can be performed using external force in order to produce optical contrast by changing the state of liquid crystal orientation between the areas of the surface protection film which are deformed by external force and the areas that are not deformed. In this technique, the memory effect is produced by inhibiting creep restoration of the surface protection film that has been deformed by external force so that the shape of the surface protection film after the application of external force has ceased remains the same as the shape produced by the application of external force.
In this technique, the memory effect is produced not by using a composite of liquid crystals and an orientation-sustaining material, but rather by inhibiting creep restoration of the surface protection film. Although creep restoration of the surface protection film is inhibited, it remains difficult for the surface protection film to retain for long periods the shape produced by the application of external force in the absence of the application of external force, so it is difficult to achieve memory sustained over extended periods of time using the technique disclosed in Japanese Laid-Open Patent Application 63-294523. Even assuming that it were possible to completely prevent creep restoration of the surface protection film and thereby to sustain memory over extended periods of time, this would in turn give rise to the problem of difficulty in erasure.